1. Field of the Invention
The present invention relates to an interpolation method and apparatus used, for example, for generating a frame signal from a field signal in televisions, video recorders, printers, photocopiers, and similar devices that use gray or color scale images in the image and data processing fields.
2. Prior Art
Pixel resolution conversion technologies have become increasingly important with the development of digital imaging devices. In IDTV (improved definition TV) and EDTV (enhanced definition TV), a single frame is generated by interlacing two fields in the broadcast signal and video signal, and the method of non-interlaced reproduction of these frames becomes very important.
This non-interlaced reproduction of frames can be easily accomplished using the information from one previous field when there is a correlation between the frames as in still images. When there is no precise frame correlation as in a moving image, the information from the previous field is the information for a point in time 1/60th second earlier and cannot be used for direct field interlacing. It is therefore necessary to interpolate the data for one field between the scan lines to reproduce one complete frame.
The printer engine in video printers and other video signal hard copy printers likewise records images with the same number of pixels as in a complete frame. If the input video signal is a still image, the printer can print the image directly to paper, but if the signal is a moving image, the printer engine must interpolate the information for one field to obtain the same number of pixels as in the full frame before printing the image.
Linear interpolation using the average values of the pixels in the preceding and following scan lines has conventionally been used for field interpolation. Because this interpolation method generates additional pixel data from only a few pixels, the object has been to smooth the image by increasing the number of pixels rather than to improve the resolution. The interpolated image is therefore relatively defocused or blurred compared with the original source image.
Another interpolation method has since been developed to resolve these problems with linear interpolation by using statistical properties of the image, e.g. the continuity between fields in a moving image, to obtain a higher vertical resolution and to obtain diagonal lines that are smoother than in the linearly interpolated image by using correlation detection.
This interpolation method using correlation detection is explained in further below with reference to FIG.
In FIG. 17 lines A and C are scan lines from the same field input continuously to the rasterizer. Line B is the scan line that is not input in this field and which must be interpolated. If the pixel to be interpolated is pixel Bn in line B where n is the pixel number, the differences (.DELTA.1, .DELTA.2, .DELTA.3) in the three brightness levels passing through pixel Bn between lines A and C are expressed by the following equations. EQU .DELTA.1=.vertline.An-1-Cn+1.vertline. EQU .DELTA.2=.vertline.An-Cn.vertline. EQU .DELTA.3=.vertline.An+1-Cn-1.vertline.
The value to be used for the interpolated pixel Bn is selected by determining which of these differences is smallest, and then applying a corresponding equation.
Thus, EQU if min.=.DELTA.1, Bn=.vertline.An-1+Cn+1.vertline./2 EQU if min.=.DELTA.2, Bn=.vertline.An+Cn.vertline./2 EQU if min.=.DELTA.3, Bn=.vertline.An+1+Cn-1.vertline./2
Thus, this interpolation method compares the level difference of the pixel An above and the pixel Cn below the interpolated pixel Bn with the level difference of the pixel An+1 right above and the pixel Cn-1 left below, and the level difference of the pixels An-1 left above and Cn+1 right below the interpolated pixel Bn. It is assumed that the continuity, i.e., correlation, between the images is highest in the direction in which the pixel level difference is minimum, and uses the average of the pixel values in this direction as the value of the interpolated pixel. (See Shashin Kogyo (Photography industry), October 1989, pp. 107-108.) There is a related method that expands this concept to gray scale interpolation and expands the direction of interpolation to the right and left of these three directions (Japanese Patent Laid-Open No. H2-177683).